What Is a “Sequencer” in the Context of an Optimistic Rollup?

A sequencer in an Optimistic Rollup is a centralized node responsible for receiving user transactions, ordering them, executing them locally, and batching them into a single transaction that is posted to the Layer 1 chain. Its centralization allows for instant transaction confirmation and efficient compression.

However, this position gives the sequencer control over transaction ordering, making it the sole source of potential MEV extraction on the L2.